1. Field of the Invention
The present invention relates to a method of producing polymers of a cationically polymerizable vinyl monomer by an improved cation polymerization. More particularly, the present invention relates to a method of producing polymers, block copolymers, terminal-functional polymers, or macromonomers whose molecular weight distribution is narrow so that the molecular weight can freely be controlled over a wide range from a low molecular weight to a high molecular weight, and whose terminal structure is also controlled freely, by cation polymerization at a controlled reaction rate.
2. Description of the Prior Arts
Generally, when cation polymerization of cationically polymerizable vinyl monomers, chain transfer reaction or chain termination occurs readily because carbenium ion which is a propagation species is unstable, and therefore it has been heretofore difficult to freely control the molecular weight of polymers and obtain polymers having narrow molecular weight distributions.
With respect to the control of the molecular weight of polymers, there are some reports on examples where attempts were made to increase the molecular weight. For example, DE 2057953 and DE 2110682 disclose that upon cation polymerization of isobutylene using a proton donating compound such as an alcohol and a Lewis acid, polymers with higher molecular weights can be obtained in the presence of compounds such as amides, esters, or pyridines in the polymerization system. However, it is difficult even in this system to control the molecular weight of the resulting polymer and obtain polymers having narrow molecular weight distributions. Moreover, the polymerization reaction is over within several tens seconds, resulting in that it is in effect impossible to fully remove heat of polymerization.
U.S. Pat. Nos. 3,994,993 and 4,276,394 describe trials for the synthesis of block copolymers. The methods disclosed therein are disadvantageous in that not only block copolymers but also respective homopolymers of comonomers used are formed in unnegligible amounts and as a result fractionation operations are required, and polymerization process is complicated.
Living polymerization, which does not cause transfer reaction or termination reaction, is easy to control the molecular weight of the resulting polymer, thus making it possible to synthesize block copolymers, to give polymers having narrow molecular weight distributions, and to control polymerization rates. Various attempts have been made in order to find a living polymerization system for cation polymerization.
Recently, there has been some reports on examples of a so-called living cation polymerization which is cation polymerization of a type in which isomerization reaction, chain transfer reaction, and termination reaction of a propagating carbenium ion are inhibited. For example, Higashimura, et al., Macromolecules, 17, 265 (1984) reported that cation living polymerization is possible in the polymerization of vinyl ether using a combination of hydrogen iodide and iodine as an initiator. However, this method has various problems, for example, that application of the polymerization with this initiator is limited to polymerization of monomers having an alkoxyl group which is highly electron donating and thus having a high cation polymerizability, and the initiator used is unstable and is difficult to handle.
On the other hand, Kennedy, et al., EP 206756 and EP 265053 have demonstrated that cation living polymerization is possible for olefin monomers by polymerizing an olefin monomer such as isobutylene using a combination of an organic carboxylic acid, ester or ether and a Lewis acid. However, this method have many problems which must be cleared before it can be used on an industrial scale.
Kennedy, et al. have used boron trichloride which has a weak polymerization activity as the Lewis acid preferentially. This is presumably because the use of a Lewis acid having a strong polymerization activity gives rise to various side reactions, which makes it difficult to control the molecular weight of the resulting polymer In fact, when use is made of titanium tetrachloride, which has a strong polymerization activity, it is difficult to control the molecular weight and polymerization rate. Generally, in cation polymerization, polymerization rate is greatly influenced by dissociation state of ion pairs of propagation species; polymerization rate decreases by the use of nonpolar solvents such as butane and pentane in which ion pairs do not dissociate. Therefore, when boron trichloride, which has a weak polymerization activity, is used as the Lewis acid, polymerization does not proceed until polar solvents such as methyl chloride are used as the solvent.
Polar solvents, which give favorable results in the method of Kennedy, et al., are a poor solvent for polyisobutylene which is produced, the polymer separates out in the system when its molecular weight reaches 5,000 or more, and the reactivity of propagation active species decreases extremely. Hence, in order to obtain high polymers with controlled molecular weights, polymerization must be carried out at extremely high rates so that the polymerization is completed before a polymer separates out. On this occasion, however, a large amount of heat is generated in a short time. Further, it is impossible to obtain block copolymers by adding monomers in succession because the polymer which are formed separate out in poor solvents.
U.S. Pat. No. 4,870,144 describes living cation polymerization performed in a mixed solvent. This method suffers from similar problems to those encountered in the method of Kennedy, et al. described above, and is not qualified as a method for the production of polymers in which the control of molecular weight is easy.